Kurzweil is a man on a mission: to save the Western world from a diet too rich in fat and calories. He believes that the current medical recommendations from such groups as the American Heart Association don't go far enough. His ``10% solution'' means limiting fat consumption to one-tenth of a day's calories and following a program of modifications to diet and life habits: restricting intake of salt and cholesterol, downing no more than two drinks a day, eschewing cigarettes and drug abuse, and making time for regular aerobic exerise and stress management. Although the AHA advises much the same, it calls only for a 30% limit of daily calories from fat. Kurzweil sets the book up as a series of questions and answers between a skeptical reader and a knowledgeable writer. One of the discussions delves into ``recent . . . research that strongly suggests we have the ability to slow down aging--to extend both life and youthfulness--through caloric reduction.'' Kurzweil suggests that one should maintain body weight at about 95% of ideal weight for a longer and more vigorous life. He invites the reader to give his program a fair trial: two months following his guidelines with a third month as the transition. He claims that attempting a major change is much easier than following moderate guidelines: ``If you are strict . . . your tastes actually change . . . But if you make occasional exceptions . . . your tastes will never change, and the new diet will continue indefinitely as a constant discipline, which will ultimately fail.'' The 10% solution makes for some fascinating reading, but seems more than the doctor would order. (Mar.)